Group template helper functions, remove Printf, improve template error messages (#23982)

Follow #23328 


Major changes:

* Group the function in `templates/help.go` by their purposes. It could
make future work easier.
* Remove the `Printf` helper function, there is already a builtin
`printf`.
* Remove `DiffStatsWidth`, replace with `Eval` in template
* Rename the `NewTextFuncMap` to `mailSubjectTextFuncMap`, it's for
subject text template only, no need to make it support HTML functions.


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And fine tune template error messages, to make it more friendly to
developers and users.


![image](https://user-images.githubusercontent.com/2114189/230714245-4fd202d1-2b25-41b2-8be5-03c5fee45091.png)


![image](https://user-images.githubusercontent.com/2114189/230714277-66783577-2a03-49d5-8e8c-ceba5e07a2d4.png)

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Co-authored-by: silverwind <me@silverwind.io>

templates/repo/migrate/migrate.tmpl

@@ -6,13 +6,13 @@
 			<div class="ui three stackable cards">
 				{{range .Services}}
 					<a class="ui card gt-df gt-ac" href="{{AppSubUrl}}/repo/migrate?service_type={{.}}&org={{$.Org}}&mirror={{$.Mirror}}">
-						{{svg (Printf "gitea-%s" .Name) 184}}
+						{{svg (printf "gitea-%s" .Name) 184}}
 						<div class="content">
 							<div class="header gt-tc">
 								{{.Title}}
 							</div>
 							<div class="description gt-tc">
-								{{(Printf "repo.migrate.%s.description" .Name) | $.locale.Tr}}
+								{{(printf "repo.migrate.%s.description" .Name) | $.locale.Tr}}
 							</div>
 						</div>
 					</a>